Reading the Rain in Rocks: A late deglacial speleothem record from Sumatra, Indonesia

Abstract

The Intertropical Convergence Zone (ITCZ) is a key component of Hadley cell circulation. In the Indo-Pacific Warm Pool (IPWP) region, the seasonal migration of the ITCZ defines much of the precipitation variability over the Maritime Continent. The seasonal migration of the ITCZ in this region is also closely related to the Australasian monsoon, which brings critical rainfall to Asia and Indo-Australia, cumulatively home to approximately 40% of the global population. On interannual timescales, rainfall in the IPWP region is also connected with zonal climate variability of the El Niño-Southern Oscillation and Indian Ocean Dipole systems. Understanding the IPWP’s climate sensitivities is therefore crucial to the improvement of long-term prediction of rainfall and drought. Abrupt changes in Atlantic Meridional Overturning Circulation (AMOC) are known to have affected the strength of the Asian monsoon during glacial and deglacial climate states. However, there is still much uncertainty around the hydroclimate response of the IPWP region to abrupt climate changes in the North Atlantic. Speleothems are powerful archives for paleoclimatic reconstruction, providing absolute-dated and often highly-resolved records of past climate. Several speleothem oxygen-isotope (δ18O) records from the central IPWP and Asian summer monsoon regions provide decadally-resolved time-series of past rainfall variability since the last glacial period. Many studies have suggested a southward shift in the ITCZ in the IPWP region during phases of reduced AMOC. However, existing IPWP proxies have seasonal biases and conflicting responses, making it difficult to determine the true extent of North Atlantic forcing in this climatically important region. In Chapter 2 of this thesis, I present a precisely-dated, high-resolution record of eastern Indian Ocean hydroclimate variability spanning the last 16 ky (thousand years) from δ18O measurements in an aragonite-calcite speleothem from central Sumatra. This represents the western-most speleothem record from the IPWP region and fills an important spatial gap in terrestrial hydroclimate, facilitating assessment of Warm Pool sensitivity at its lateral extent. Petrographic and geochemical analysis reveals that the sample is principally composed of aragonite but is punctuated by intervals of primary calcite growth. In addition to mineralogical determination by Raman spectroscopy, trace element analysis by laser ablation ICP-MS reveals strongly antiphased behaviour between magnesium and strontium attributed to the strong preference of those elements for the calcite and aragonite lattices, respectively. In Chapter 4, this relationship is utilized to develop a quantitative correction for the stable isotope fractionation offset between the two calcium carbonate polymorphs identified in the speleothem and to quantify partitioning coefficients for those elements into aragonite. The corrected δ18O record demonstrates a clear deglacial structure that includes 18O enrichment during the Younger Dryas (~12.9-11.7 ka; thousand years ago) and 18O depletion during the Bølling-Allerød (~14.7-12.9 ka), similar to the pattern seen in speleothems of the Asian and Indian monsoon realms. In contrast, other speleothem records from the IPWP show slight increases or no change in δ18O during the Younger Dryas. To better interpret the spatial pattern of speleothem δ18O change during the Younger Dryas, Chapter 3 uses back-trajectory air parcel analysis to identify primary moisture source regions and seasonal distributions of moisture to IPWP speleothem sites. This information is evaluated alongside modelled GISS ModelE-R vapour source distributions to evaluate how moisture sources may have changed during simulated hosing events analogous to the Younger Dryas. Chapter 4 considers the environmental controls that could account for the mineralogy of the Sumatran speleothem sample. Shifts between aragonite and calcite phases in the speleothem are partly driven by environmental variability, with aragonite associated with drier phases like the Younger Dryas and calcite generally associated with increased detrital material. However, these changes are more likely to be related to cave hydrology and filtration rates than directly reflective of rainfall amounts. Together, this research establishes a robust basis for interpreting the climatic history retained in the 16 ky speleothem δ18O record for Tangga Cave and provides context relative to other speleothem records from the Indo-Pacific Warm Pool region. This work demonstrates the extended reach of North Atlantic abrupt forcing into the eastern tropical Indian Ocean, supporting this conclusion with systematic analysis of the modern hydroclimate system through use of isotope enabled climate models and back-trajectory air parcel analysis.